1. Field of the Invention
The present invention relates generally to a vibration sensor for determining the magnitude of vibration. More specifically, the invention relates to a vibration sensor for determining the magnitude of vibration of an internal combustion engine, and which uses a vibrator including a piezoelectric vibration element.
2. Description of the Prior Art
Recently, vibration sensors have been used on automotive vehicles for sensing and determining the magnitude of engine vibration. In a spark ignition timing control system for the internal combustion engine, the engine vibration is one of the important parameters.
Generally, engine knocking will cause shortening of engine life, and particularly, when substantial knocking continues, the engine may be permanently damaged. However, from the view point of fuel economy and maximization of engine output characteristics, it is desirable to run the engine in a lightly knocking condition. As is well-known to those skilled in the art, for causing knocking of the internal combustion engine, the spark advance angle is altered. In general, engine knocking increases in correspondence with increases in the spark angle advance. Therefore, controlling the knocking condition can be accomplished by controlling spark advance angle. In the prior art, there have been developed and proposed various systems for keeping the internal combustion engine in a lightly knocking condition by feedback controlling the spark advance angle.
On the other hand, it is also well known that under the knocking condition, engine vibration rapidly increases depending on the internal pressure in the combustion chamber. In currently made vehicles, the engine vibration frequency corresponding to the knocking condition is generally within a range of 6 to 9 KHz. Utilizing this engine characteristic, there have been proposed various systems for detecting engine knocking condition by detecting the engine vibration. For detecting the engine vibration, there has been provided a vibration sensor for the internal combustion engine, which detects the above-mentioned specific range of engine vibration. The detected vibration frequency signal is converted into an analog value which is smoothed and compared with the vibration element of the specific frequency range to generate a signal. The signal is integrated per crank revolution. When the integrated value exceeds a predetermined value, the engine is regarded as knocking and the system generates an output knocking signal. Corresponding to this knocking signal, the spark advance angle is feedback controlled.
In the prior art, vibration sensors have been used for detecting knocking condition of the internal combustion engine. In a typical vibration sensor construction, the vibration sensor comprises a sensor body defining an internal space with a cover member and a vibrator disposed in the internal space of the sensor body. The vibrator generally comprises a metal plate and a piezoelectric vibration element which generates an electric signal corresponding to the vibration applied thereto. Conventionally, the vibrator is bonded onto a vibrator base formed on either the sensor body or the cover member. For securing the vibrator onto the vibrator base, an electrically conductive adhesive is used. The vibrator is connected to the control system, such as the spark ignition timing control system, through a lead inserted into the internal space of the vibrator body. The end of the lead is secured to the vibrator by welding or a similar method.
The sensor thus constructed is secured to an engine cylinder block with an achor bolt formed integrally with the sensor body. The electric signal generated in the vibrator is transmitted to the control system through the lead.
Since the electrically conductive adhesive has a relatively low heat resistance, under extreme heat conditions, it breaks down, causing the vibrator to be jarred loose from the vibrator base. In addition, the relatively high temperature will affect the vibrator's resonant frequency and decrease the vibrator's durability.
Further, the end of the lead is welded or soldered to the piezoelectric element with relatively high heat. This will possibly damage the piezoelectric element and therefore makes it difficult to fit the lead onto the piezoelectric element.